The present invention provides an isolated nucleic acid including a nucleotide sequence encoding COL8, which controls hypocotyl length and flowering time in a plant. It is found that plants over-expressing COL8 under long day conditions show slight hypocotyl elongation and delayed flowering in comparison with the wild type. In addition, COL8 and FKF1, which is a circadian-clock related gene, are found to be localized at the same site in a plant cell, indicating the existence of an interaction between the two.